Polyimides ("PI") are a group of high polymers in which repeating units are connected by imide groups (--(CO).sub.2 N--) in the polymer chain. Polyimides have good tensile strength, low water absorption, remain undistorted by heat at temperatures of 260.degree. C. or higher, have a low dielectric constant, and a relatively small coefficient of linear expansion. They also exhibit high temperature stability (up to about 370.degree. C.) excellent frictional characteristics, good wear resistance at high temperatures, radiation resistance, low outgassing under high vacuum, are resistant to organic materials at relatively high temperatures, and are flame retardant. Therefore, polyimides have found a number of uses including high temperature coatings, laminates and composites for aerospace vehicles, ablative materials, oil sealants and retainers, adhesives, semiconductor applications, valve seats, bearings, insulation for cables, printed circuits, magnetic tapes, flame resistant fibers, and binders for abrasive materials; (N. Sax and R. Lewis, Hawley's Condensed Chemical Distionary, 11th Ed., 1987).
In the microelectronics or semiconductor industry, polyimides have found a number of applications including the formation of planarizing dielectric films between layers. As known to those familiar with microelectronic integrated circuit (or "chip") manufacturing, the steps used to build up a microelectronic device typically result in a three-dimensional surface pattern of semiconductor materials, insulators and conductors, among other materials. These three-dimensional topographies are often referred to as "geometries" in this art. Although a resulting finished semiconductor device will exhibit such geometries, during manufacturing, when the various layers of materials are being added, there are some processes for which the three-dimensional structure is disadvantageous. In particular, during typical photolithographic steps, a planar surface is much more desirable than a three-dimensional one because of the high optical resolution required during such steps. A uneven surface can lower the resolution of a photolithographic step, while a highly planar one can help increase the resolution. The ability to achieve planar surfaces over varying topography is critical to lithographic steps which require etching small features; i.e. those of about one micron or smaller.
Therefore, one typical step during semiconductor device manufacturing is to add a layer of polyimide which fills in and levels out the geometries, a process referred to as "planarizing." The polyimide, to do the best job of planarizing, is generally added as a film of a solution or suspension in which a high solids content and low viscosity are generally accepted as favoring improved planarization properties. The film is then thermally cured to produce the desired polyimide layer.
In the polymer art, the term "cure" is often used to represent two different process steps. In one sense, "cure" represents the chemical reaction in which particular reactants are used to produce a resulting polymer. An example is the production of polyimide in a solution. The word "cure" is also used, however, to represent the process by which a polymer in solution is formed into a solid polymer, usually by driving off a solvent along with water and other reaction products. An example is the solidification of a polyimide solution into a solid film. In order to distinguish these steps, the solvent removal or similar steps will be referred to herein as a "thermal cure," reflecting the common use of heat to accomplish this step.
As also used herein, the term "film" refers to a very thin liquid coating, usually of a solution, upon a substrate. In the manufacture of microelectronic devices, such films are often applied by applying the solution to a rapidly rotating substrate, a technique that helps form an even film. The technique is accordingly referred to as "spin casting" or "spinning" and the resulting film as a "spun" or "spun cast" film. The term "film," however, generally is used to refer to the coating both in its liquid state and after it has been hardened into a resulting solid.
Typical procedures for preparing polyimide films from spun cast polyamic acid films, however, involve complex processes over extended periods of time with temperatures that reach as high as 400.degree. C. or greater. In some chip manufacturing processes, however, such high temperatures are disadvantageous, particularly with respect to their effects on the other materials present, and the deposit and cure of polyimide at relatively low temperatures would offer significant advantages.
One attempt to deliver polyimide at lower temperatures is to supply polyimide as a low molecular weight polyimide powder dissolved (or redissolved) in one of the common polyimide solvents such as 1-methyl-2-pyrolidinone (NMP). The resulting solution is used in the spinning techniques familiar to those in this art to add polyimide films of a desired thickness. Films formed from these low molecular weight powders dissolved in a solvent or mixture of solvents can be thermally cured at relatively lower temperatures of about 300.degree. C. Unfortunately, however, the mechanical and electrical properties of films formed from such low molecular weight powders are inferior in many respects. They are readily attacked by many common solvents, tend to be susceptible to moisture, exhibit poor adhesion, and have poor dielectric properties.
Alternatively, in order to achieve improved mechanical and electrical properties, the polyimide is best added as a solution of polyamic acid. As used herein, the term "polyamic acid" refers to a polyimide precursor and generally represents a polymer having an amide bonding scheme which, when the amide linkage is dehydrated, will form an imide linkage. Because the amide is supplied in polymeric form, the dehydration step produces polyimide. Films formed from polyamic acid solutions generally demonstrate better mechanical and electrical properties than do films formed from solutions of polyimide powders. Unfortunately the polyamic acid must be subjected to a complex curing process which typically includes a 30 minute treatment at a temperature of about 400.degree. C. or greater to achieve the required imidization or water removal step.
Therefore, investigators have pursued the problem of producing polyimides suitable for microelectronic processes that can be produced or cured at generally lower temperatures. Some of these have included chemical curing of polyimides using phosphorous halide and similar curing agents. These procedures, however, introduce impurities into the resulting polyimide films and produce unusable materials.
Nevertheless, the potential benefits of curing polyimides at lower temperatures are attractive and offer several advantages over redissolving polyimide powder in solvents. First, the molecular weights of polyamic acids tend to be greater than those of redissolved polyimide powders. Thus, by chemically curing polyamic acid the original high molecular weight polyamic acid can be converted into a high molecular weight polyimide. Second, because the chemically cured solutions contain polyimide chains rather than polyamic acid chains, high molecular weight polyimide films spun from these solutions ought be curable at temperatures somewhat lower than that of the conventional curing scheme, depending upon the solvent system. Third, the mechanical and electrical properties of high molecular weight polyimide films are desirable, particularly if they could be made comparable to those formed from the polyamic acid precursor. These would desirably be superior to those of films formed from polyimide solutions prepared by dissolving the low molecular weight PI powder.
Therefore, it is an object of the present invention to provide a method of preparing and curing low viscosity, highly planarizing polyimides at moderate temperatures, and for which the resulting polyimides are resistant to attack from common organic solvents and are particularly suitable for use in microelectronic applications.
In one aspect, the invention is a method of preparing and curing low viscosity, highly planarizing polyimides at moderate temperatures for use in microelectronic applications. In this aspect the invention comprises curing polyamic acid into polyimide by adding a hydrophilic reagent to a polyamic acid solution, and in which the hydrophilic reagent has little or no reactivity with amines or carboxylic acids and that removes the water normally produced by the imidization of polyamic acid to polyimide to thereby encourage the formation of polyimide in the solution. As a result, the associated removal of water drives the polyamic acid dehydration reaction to produce polyimide at moderate temperatures.
In another aspect, the invention is a precursor solution from which a low viscosity, highly planarizing polyimide film can be formed at moderate temperatures.
In yet another aspect, the invention is a method of planarizing the surface of a microelectronic device such as an integrated circuit or its precursor.
In a further aspect, the invention is a precursor for a microelectronic device such as an integrated circuit.
The foregoing and other objects, advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings in which: